An external preparation in which a drug containing a hydrophilic drug as a principal agent and having been absorbed through skin exhibits systemic action when transferred to the systemic circulation (a systemic action type external preparation) and an external preparation in which a drug absorbed through skin locally shows efficacy (a local action type external preparation) are used.
It is known that a hydrophilic drug having a comparatively low molecular weight in particular tends to be easily permeable through skin in general. Such a hydrophilic drug having a low molecular weight has, however, a problem that although high efficacy can be attained immediately after starting administration, a principal agent contained in the preparation is so early depleted that the efficacy does not last. Therefore, for example, for purposes of long-term administration, a technique for stabilization by changing a specific basic drug to a molecular type has been proposed (Patent Literature 1).
Patent Literature 2 discloses an S/O (Solid-in-Oil) type external preparation in which a drug-containing complex is dissolved or dispersed in an oil phase. It is described that the drug-containing complex is in the form of a solid in which a hydrophilic drug is covered by a surfactant. Patent Literature 2 describes that such an S/O type external preparation is excellent in transdermal absorption.
It is, however, known that a hydrophilic drug having a comparatively high molecular weight tends to be difficult to permeate through skin in general.